How to make water tight surface for 3D printing
The thickness you're using for Solidify modifier is 2, that means two blender units, and you're not using clamp, the screenshot's don't show the scale you're using, but just to give you a hint, if you're using meters for instance, you're using 2 meters for thickness.
You can use Offset to change the direction of extruding faces using this modifier, but to be honest, looking at the mesh, I don't believe it's possible to get good result using Solidify, since this will be applied to the whole mesh, so you have to reduce the thickness not to exceed the closest faces to each other, the bolts like faces are too close to each others compared to the scale of the whole mesh.
The best method is to copy the object, make the copy smaller, decimate the mesh to a simpler version, this might need some manual tweaking to make sure no internal faces or overlapping is taking place, then position it in place inside the original mesh, and weld the two meshes together using Ctrl + J